Power supplied to an integrated circuit (IC) occurs through a power distribution network. The power distribution network starts with a power supply that generates an appropriate DC voltage. The power supplied to the IC must traverse from the power supply across the power distribution network before the power reaches the IC. The power distribution network has characteristics that may affect the operation of the IC.
FIG. 1 shows a conventional printed circuit board system (10). The printed circuit board system (10) includes a printed circuit board (PCB) (12). The PCB (12) is a central platform on which various components are mounted. The PCB (12) has multiple layers that contain traces that connect the power supply and signals to the various components mounted on the PCB (12). Two layers, a system power supply layer (14) and a system ground layer (16), are shown in FIG. 1.
The system power supply layer (14) and the system ground layer (16) provide power to an IC (20). The power supplied to the IC (20) traverses the system power supply layer (14) and the system ground layer (16) from a DC source (not shown) to a package (18) on which the IC (20) is mounted. Other components are also mounted on the PCB (12) that generally attempt to maintain a constant voltage supplied to the IC (20). These components may include, but are not limited to, an air-core inductor (24), a power supply regulating integrated circuit (26), switching transistors (28), a tantalum capacitor (30), and electrolytic capacitors (32).
A variety of different types and different locations of capacitors are used to help maintain a constant voltage supplied to the IC (20). Electrolytic capacitors (32) mounted on the PCB (12) connect between the system power supply layer (14) and the system ground layer (16). The package (18), similar to the PCB (12), may include multiple planes and interconnections between the planes to provide a connective substrate in which power and data signals traverse. Ceramic capacitors (22) mounted on the package (18) connect between a package power supply signal (not shown) and a package ground signal (not shown).
Due to active switching of circuit elements on the IC (20), the power required by the IC (20) changes. The active switching causes power supply noise. Additional components may be included to minimize such power supply noise. For example, ceramic capacitors (22) near the IC (20) act as local power supplies by storing and dissipating charge as needed.
The addition of components reduces the power supply impedance at most frequencies; however, localized impedance peaks may exist. The impedance peaks indicate a power supply resonance. The power supply resonance is formed when parasitics in the power distribution network and components connected to the power distribution network are excited at a particular frequency. The parasitics include the inherent inductance, resistance, and capacitance that may exist in the IC (20) (or other integrated circuits), the package (18), and the power distribution network. In particular, the power supply resonance may be formed from the power distribution network and a “parasitic tank circuit” that includes the IC capacitance and package inductance.
FIG. 2 shows a schematic of a power distribution network for a package (296). The power distribution network is represented by impedances Z1 (204), Z2 (206), and Z3 (208). Each of these impedances (204, 206, 208) may include resistive, inductive, and/or capacitive elements. Two power supply lines (292, 294) supply power to the package (296) located between the two power supply lines (292, 294). The impedances (204, 206, 208) model both the inherent parasitics of the power distribution network and added components.
On the package (296), various forms of capacitance may be used to further stabilize the power supply. Low equivalent series resistance (ESR) local decoupling capacitors are modeled by resistor (262) and capacitor (264). High ESR global decoupling capacitors are modeled by resistor (266) and capacitor (268). Non-switching logic disposed on an IC in the package (296) is modeled by resistor (270) and capacitors (272, 274). Switching logic disposed on the IC in the package (296) is modeled by variable resistors (276, 278) and capacitors (280, 282).
In FIG. 2, the schematic of the power distribution network may be used to simulate the power supply impedance observed by the IC in the package (296), as represented by “Z.” To measure the power supply impedance, a 1 Ampere AC current source (290) injects current onto power supply line (292). The measured voltage, VM, between the two power supply lines (292, 294) may be used to calculate the power supply impedance. The impedance Z is equal to VM divided by the 1 Ampere AC current source (290). By varying the frequency of the 1 Ampere AC current source (290), a frequency versus impedance relationship may be determined.
A representative graph of power supply impedance is shown in FIG. 3. Over a particular range of frequencies for the switching logic on the IC (296), the power supply impedance increases because the circuit formed by an IC and a package resonates. A spike in a power supply impedance curve (302), or a power supply resonance frequency, has the effect of current-starving the IC in the package (296 in FIG. 2). When the IC is current-starved, some voltage potentials on the IC in the package (296 in FIG. 2) may shift from their desired values. Accordingly, an increase in the power supply impedance may cause undesired operation of the IC in the package (296 in FIG. 2).